WASHINGTON — When President-elect Donald J. Trump offered Rick Perry the job of energy secretary five weeks ago, Mr. Perry gladly accepted, believing he was taking on a role as a global ambassador for the American oil and gas industry that he had long championed in his home state.

In the days after, Mr. Perry, the former Texas governor, discovered that he would be no such thing — that in fact, if confirmed by the Senate, he would become the steward of a vast national security complex he knew almost nothing about, caring for the most fearsome weapons on the planet, the United States’ nuclear arsenal.

Two-thirds of the agency’s annual $30 billion budget is devoted to maintaining, refurbishing and keeping safe the nation’s nuclear stockpile; thwarting nuclear proliferation; cleaning up and rebuilding an aging constellation of nuclear production facilities; and overseeing national laboratories that are considered the crown jewels of government science.

“If you asked him on that first day he said yes, he would have said, ‘I want to be an advocate for energy,’” said Michael McKenna, a Republican energy lobbyist who advised Mr. Perry’s 2016 presidential campaign and worked on the Trump transition’s Energy Department team in its early days. “If you asked him now, he’d say, ‘I’m serious about the challenges facing the nuclear complex.’ It’s been a learning curve.”

Mr. Perry, who once called for the elimination of the Energy Department, will begin the confirmation process Thursday with a hearing before the Senate Energy Committee. If approved by the Senate, he will take over from a secretary, Ernest J. Moniz, who was chairman of the Massachusetts Institute of Technology physics department and directed the linear accelerator at M.I.T.’s Laboratory for Nuclear Science. Before Mr. Moniz, the job belonged to Steven Chu, a physicist who won a Nobel Prize.

For Mr. Moniz, the future of nuclear science has been a lifelong obsession; he spent his early years working at the Stanford Linear Accelerator Center. Mr. Perry studied animal husbandry and led cheers at Texas A&M University.

Mr. Moniz had such deep experience with nuclear weapons that in 2015, President Obama made him a co-negotiator, along with Secretary of State John Kerry, of the Iran nuclear deal.

Mr. Perry would sit atop the men and women making the judgments about whether Iran is complying with that accord. In the basement of the Energy Department’s headquarters, the agency’s intelligence unit monitors compliance, working closely with the C.I.A., the National Security Agency and other intelligence bodies.

While even Mr. Perry’s supporters concede that he has no experience making high-level decisions on nuclear weapons policy, he has had some dealings with the problem of nuclear waste, which also falls under the purview of the Energy Department.

As governor, Mr. Perry pushed a plan to create a low-level nuclear waste repository in Texas, to be privately run by a Dallas-based company, Waste Control Specialists, which was also a contributor to Mr. Perry’s re-election campaigns. The question of what to do with nuclear waste has long been a central problem for the Energy Department. By law, it is supposed to be buried at a federally run repository to be built at Yucca Mountain in Nevada.

“No other state has licensed a nuclear waste facility like this, and it was all done on Governor Perry’s watch,” said Charles McDonald, a spokesman for Waste Control Specialists. “He really understands this stuff.”

Mr. Perry’s backers also note that Texas is home to Pantex, an Energy Department plant where nuclear weapons are assembled. But as governor, he had no role in running the facility.

His supporters say, however, that his experience running the nation’s second-largest state economy has sufficiently prepared him. “Like with any of these positions in Washington, what is needed is strong leadership skills,” Deirdre Delisi, his former chief of staff, said. “I have no doubt he will be able to attract the best and the brightest.”

In a recent interview in the energy secretary’s office, down the hall from the secure facility where he gets his updates on nuclear threats, Mr. Moniz talked about the challenges the United States will face as the Trump administration decides where to take a multibillion-dollar renovation of the nation’s nuclear production facilities and laboratories. Mr. Moniz championed that effort, hoping it would position the United States to preserve its nuclear force without resuming nuclear tests, and ultimately to reduce the number of nuclear warheads.

“We refurbished our weapons to make them safer and more reliable,” Mr. Moniz said, choosing his words with precision. “We didn’t ‘modernize.’” Modernization, he said, “is what Russia is doing and China is doing,” which, he acknowledged, could jeopardize Mr. Obama’s vision of a world free of nuclear weapons.

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Ernest J. Moniz, center, was chairman of the M.I.T. physics department before becoming energy secretary under President Obama.CreditAl Drago/The New York Times

But the scope of the Obama-era effort was so broad that, paradoxically, it will give Mr. Perry and Mr. Trump the opportunity to go in a very different direction: With the scientific and infrastructural upgrades, America is now better poised to resume an arms race that Mr. Trump recently said he was open to pursuing.

If Mr. Perry has views on those issues — including on whether to test nuclear weapons rather than build computer models of how they would perform — they are unknown.

He would not be the first nonexpert to run the Energy Department. Bill Richardson took the job under President Bill Clinton after serving as ambassador to the United Nations, and later became the governor of New Mexico.

But Mr. Perry’s qualifications to oversee a muscular renovation, or expansion, of the nation’s nuclear weapons complex are expected to be among the chief topics of questioning at his confirmation hearing. Mr. Trump’s transition team declined a request to interview Mr. Perry.

“Rick Perry was pitch-perfect for Texas politics,” said Calvin Jillson, a professor of political science at Southern Methodist University in Texas. “He has very close ties to the oil industry. He is about ‘the Texas way’ — low taxes, low regulation. But none of that gives him the depth of knowledge needed for running the Energy Department.”

Mr. Perry is attuned to that vulnerability. The Energy Department was on the list of agencies he said he wanted to eliminate when he ran for the Republican presidential nomination in 2012 — though he famously forgot its name during a debate. Despite what he called his “oops” moment, he stood by his call to dismantle the department, saying, “They’ve never created one bit of energy, the best I can tell.”

If confirmed, Mr. Perry would be at the table for one of the first big debates of the Trump presidency: what to do with the Iran nuclear deal that Mr. Moniz played such a critical role in shaping. Mr. Trump has called the deal a “disaster,” and Vice President-elect Mike Pence talked during the campaign about scrapping it.

But the president-elect’s pick for defense secretary, Gen. James N. Mattis, advised keeping the agreement during his confirmation hearing, and the nominee for secretary of state, Rex W. Tillerson, was equally cautious.